CONICET | Buscador de Institutos y Recursos Humanos

Magnetorheological elastomers, MREs, based on elastic organic matrices displaying anisotropic magnetoresistance and piezoresistivity at room temperature were prepared and characterized. These materials are dispersions of superparamagnetic magnetite forming cores of aggregated nanoparticles inside silver microparticles that are dispersed in an elastomeric polymer(poly(dimethylsiloxane), PDMS), curing the polymer in the presence of a uniform magneticﬁeld. In this way, the elastic material becomes structured as the application of theﬁeld induces the formation ofﬁlaments of silver-covered inorganic material agglomerates (needles) aligned in the direction of theﬁeld (parallel to theﬁeld). Because the magnetic particles are covered with silver, the MREs are not only magnetic but also electrical conductors. The structuration induces elastic, magnetic, and electrical anisotropic properties. For example, with a low concentration of particles in the elastic matrix (5% w/w) it is possible to obtain resistances of a few ohms whenmeasured parallel to the needles or several megaohms in the perpendicular direction. Magnetite nanoparticles (Fe3O4 NP) were synthesized by the coprecipitation method, and then agglomerations of these NPs were covered with Ag. The average size of the obtained magnetite NPs was about 13 nm, and the magnetite-silver particles, referred to as Fe3O4@Ag, form micrometric aggregates (1.3ìm).Nanoparticles,microparticles,andtheMREswerecharacterizedbyXRD,TEM,SEM,EDS,diﬀuse reﬂectance, voltammetry,VSM, and SQUID. At room temperature, the synthesized magnetite and Fe3O4@Ag particles are in a superparamagnetic state (TB = 205 and 179 K at 0.01 T as determined by SQUID). The elastic properties and Young?s modulus of the MREs were measured as a function of the orientation using a texture analysis device. The magnetic anisotropy in the MRE composite was investigated by FMR.The electrical conductivity of the MRE (ó) increases exponentially when a pressure,P, is applied, and the magnitude of the change strongly depends on what directionP is exerted (anisotropic piezoresistivity). In addition, at aﬁxed pressure,ó increases exponentiallyinthepresenceofanexternalmagneticﬁeld (H)onlywhentheﬁeldH is applied in the collinear direction with respect to the electrical ﬂux, J. Excellentﬁts of the experimental dataó versus H andP were achieved using a model that considers the intergrain electron transport where an H-dependent barrier was considered in addition to the intrinsic intergrain resistance in a percolation process. The H-dependent barrier decreases with the appliedﬁeld, which is attributed to the increasing match of spin-polarization in the silver covers between grains. The eﬀect is anisotropic (i.e., the sensitivity of the magnetoresistive eﬀect is dependent on the relative orientation betweenH and the currentﬂowJ). In the case of Fe3O4@Ag,whenH andJ are parallel to the needles in the PDMS matrix, we obtain changes inó up to 50% forﬁelds of 400 mT and with resistances on the order of 1−10Ù. Magnetoresistive and magnetoelastic properties make these materials very interesting for applications inﬂexible electronics, electronic skins, anisotropic pressure, and magneticﬁeld sensors

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